[styling] Consider enabling CSS styling for the site banner

[springcomp]

This is part of a series or minor issues that aim to enable support for flexible styling and theming of the JMESPath site.

It does not seem to be possible to style the entire width of the site header.
Please, consider enabling CSS styling for the site banner.

[innovate-invent]

The page margin is set to 10% on each side: https://github.com/jmespath-community/jmespath.site/blob/80b63b747dd9b5d52c8f9b7c0b5fc67777b22be1/themes/jmespath/assets/main.scss#L2-L4

To expand the header into the page margin you set the header margin to a negative value:
https://github.com/jmespath-community/jmespath.site/blob/80b63b747dd9b5d52c8f9b7c0b5fc67777b22be1/themes/jmespath/assets/main.scss#L22-L23

[innovate-invent]

I should also add that I was planning at some point to create some @media rules to handle mobile and print format.